There are switches each having a first main terminal, a second main terminal, and a main control terminal, such as MOSFETs or IGBTs. Such a switch is turned on when a potential difference of the main control terminal relative to the second main terminal becomes equal to or higher than a threshold voltage; the on state of the switch allows a current to flow between the first and second main terminals. In contrast, the switch is turned off when the potential difference of the main control terminal relative to the second main terminal becomes lower than the threshold voltage; the off state of the switch prevents a current from flowing between the first and second main terminals.
Such a switch has a feedback capacitance formed between the first main terminal and the main control terminal. While the switch is in the off state, electrical charge is supplied to the main control terminal via the feedback capacitance. This may result in the potential difference of the main control terminal relative to the second main terminal becomes equal to or higher than the threshold voltage. This therefore may result in self-turn-on of the switch; the self-turn-on of the switch causes the switch to be erroneously switched from the off state to the on state although the switch should be maintained in the off state.
For addressing this issue, PCT International Publication No. 2016/030954 discloses a drive circuit that applies a negative voltage to the main control terminal of a target switch to thereby reduce the occurrence of self-turn-on of the switch. Specifically, the drive circuit disclosed in the published patent document includes first and second switches each being an N-channel MOSFET, and a third switch being a P-channel MOSFET. The drive circuit also includes a first negative voltage source that outputs a first negative voltage, such as −15 V, and a second negative voltage source that outputs a second negative voltage, such as −5V, that is higher than the first negative voltage. Note that, in this specification, “lower” means “more negative” when applied to a negative voltage, and “higher” means less negative or more positive.
The drive circuit is configured to selectively apply the first and second negative voltages to the main control terminal of the target switch to thereby prevent self-turn-on of the target switch. This configuration aims to retard deterioration of the target switch due to continuous application of a large voltage to the main control terminal of the target switch.
The following further describes the drive circuit disclosed in the published patent document. The source of the first switch is connected to the first negative voltage source, and the drain of the first switch is connected to the main control terminal of the target switch. The drain of the second switch is connected to the main control terminal of the target switch, and the source of the second switch is connected to the drain of the third switch. The source of the third switch is connected to the second negative voltage source. While the first switch is in the on state, the intrinsic diode of the third switch prevents interference between the first and second negative voltage sources.